The present invention relates to an antilock brake control method and system for motor vehicles.
In prior art hydraulic braking system for motor vehicles, there have been proposed and demonstrated various types of antilock brake control devices in which when the brakes are applied to cause the braking hydraulic pressure to be supplied to the brake device for each wheel and the hydraulic pressure in the brake device is increased, the hydraulic pressure in the brake device is decreased in response to drop of the wheel speed, and in which the hydraulic pressure is increased again after recovery of the wheel speed due to the decrease in hydraulic pressure in the brake device, and the same control pattern is repeated whereby effective braking operation can be accomplished. Such a type of antilock brake control device is disclosed in Japanese Patent Application Laid-Open Publication (Kokai) No. 60-61354 published Apr. 9, 1985.
In the case of conventional antilock brake devices of the type described above, control is carried out for decreasing and reincreasing the braking hydraulic pressure in response to wheel-speed signals delivered from wheel speed sensors provided for respective wheels. In practice, the hydraulic brake line system is formed as a dual brake line system in which two pairs of diagonally opposing wheels are connected to two independent brake lines, respectively, so that even in case of failure of one brake line, an emergency braking force can be produced by the other brake line. Generally, in this case, the lower wheel speed of the speeds of the two wheels connected to the same brake line is selected, and in response to the selected lower wheel speed, the braking hydraulic pressure control in that brake line is carried out so that all the wheels are prevented from locking.
However, in the case of a J-turn (which is a well known state when a vehicle enters a curve at a speed exceeding a predetermined value and the driver makes a sharp turn), there occurs the following phenomenon. That is, when the antilock brake device operates in the manner described above, the difference in wheel speed between the inner and outer wheels becomes large and lateral acceleration is produced so that the inner wheels tend to rise up and the reaction forces from the surface of the road acting on the inner wheels are consequently decreased. As a result, the wheel speeds of the inner wheels are considerably decreased when compared with the wheel speeds of the outer wheels, and consequently the control for decreasing and increasing the braking hydraulic pressure is carried out mainly on the basis of the wheel speeds of the inner wheels. Then, since the reaction forces from the surface of the road acting on the inner wheels are low as described above, the wheel speeds are abruptly decreased in response to a slight increase in the hydraulic pressure, and consequently the wheels are brought to a state in which the wheels are almost locked. Even when the hydraulic pressure is decreased under such conditions, it takes a long time for the wheel speeds to recover, and the "no braking" mode (the state in which the hydraulic pressure is decreased) becomes longer. As a consequence, the braking force becomes insufficient as a whole, so that there arises the problem that the distance through which the vehicle travels after braking becomes longer than expected in an extremely dangerous state in which the brakes are abruptly applied in the J-turn state, which seriously inhibits the safety drive.